The present invention relates to a nonaqueous electrolyte type secondary battery including an electricity generating element accommodated in a film casing and more particularly to a nonaqueous electrolyte type secondary battery configured to swell little.
A secondary battery of the type described is conventional and includes an electricity generating element and a casing accommodating the electricity generating element. The casing is implemented by two aluminum laminate films each consisting or aluminum foil and a thermo-bonding resin film formed on the aluminum foil. Metallic leads protrude from the inside to the outside of the casing via the bonded portion of the casing. Today, there is an increasing demand for a thin, light battery configuration capable of implementing a thin, light electric apparatus. In this sense, the battery accommodated in the film casing is advantageous over a battery accommodated in a hard metallic casing.
The battery with the film casing needs more strict safety implementations than the battery with the hard metallic casing. For example, the battery should undergo a minimum of deformation in contour. Of course, the battery with the film casing should have an ability generally required of a battery, e.g., high energy density (high charge-discharge capacity), a high cycle characteristic, and a storage capacity characteristic despite self-discharge.
Lithium manganate is attracting increasing attention as one of positive electrode substances for a lithium ion secondary battery. Lithium manganate has a spinel structure represented by LiMn2O4 and functions as a 4V class, positive electrode substance in relation to a xcex-MnO2 composition. Lithium manganate with the spinel structure has a tridimensional host structure different from a layer structure particular to, e.g., LiCoO2, so that most of stoichiometric capacity available therewith can be used. Lithium manganate is therefore expected to have a desirable cycle characteristic.
Further, lithium manganate with the spinel structure allows lithium ions to be pulled out while maintaining its basic frame. This compound therefore starts releasing oxygen at a higher temperature than cobalt acid lithium having a layer, halite structure and is expected to be desirable from the safety standpoint. The safety feature is particularly important when it comes to the battery with the soft film casing- In practice, however, a lithium secondary battery including a positive electrode implemented by lithium manganate has a problem that its capacity decreases little by little due to repeated charging and discharging. This problem is serious in the aspect of practical use.
Various schemes have heretofore been proposed to improve the cycle characteristic of an organic electrolyte type secondary battery whose positive electrode is implemented by lithium manganate. For example, Japanese Patent Laid-Open Publication Nos. 3-67464, 3-119656, 3-127453, 7-245106 and 7-73883 teach improvements achievable by improving reactiveness at the time of production. Also, Japanese Patent Laid-Open Publication Nos. 4-198028, 5-28307, 6-295724 and 7-97216 teach improvements achievable by controlling a grain size. Further, Japanese Patent Laid-Open Publication No. 5-21063 teaches an improvement attainable by removing impurities. None of such schemes, however, achieves a satisfactory cycle characteristic.
Japanese Patent Laid-Open Publication No. 2-270268 proposes to improve the cycle characteristic by selecting an Li composition ratio sufficiently greater than a stoichiometric ratio. This kind of scheme is disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 4-123769, 4-147573, 5-205744 and 7-282798 also. Experiments actually indicated the improvement in cycle characteristic achievable with such a scheme.
Japanese Patent Laid-Open Publication Nos. 6-338320 and 7-262984, for example, each use LiMn2O4, which is a Mn spinel substance, and Li2Mn2O4, LiMnO2, Mi2MnO3 or similar Li-Mn compound oxide, which is richer than the above Mn spinel substance, as a positive electrode active substance. However, adding excessive Li or mixing it with another Li-rich compound reduces the charge-discharge capacity and charge-discharge energy although improving the cycle characteristic. As a result, high energy density and long cycle life are not compatible with each other. By contrast, Japanese Patent Laid-Open Publication No. 6-275276 proposes to increase a specific surface area for achieving a high rate, charge-discharge characteristic (great current relative to capacity at the time of charging and discharging) and perfect reaction. This, however, obstructs an increase in cycle life.
On the other hand, it has been studied to improve the characteristics by adding another element to a Li-Mn-O compound. For example, Japanese Patent Laid-Open Publication Nos. 4-141954, 4-160758, 4-169076, 4-237970, 4-282560, 4-289662, 5-28991 and 7-14572 each propose to add or dope, e.g., Co, Ni, Fe, Cr or Al. However, the addition of such a metal element reduces the charge-discharge capacity and needs further studies to satisfy the total ability.
As for the addition of another element, boron is expected to improve the other characteristics, e.g., cycle characteristic and self-discharge characteristic while degrading the charge-discharge characteristic little. This described in, e.g., Japanese Patent Laid-Open Publication Nos. 2-253560, 3-297058, and 9-115515. In any case, a manganese dioxide or a lithium-manganese compound oxide is mixed with a boron compound (e.g. boric acid) in a solid phase or immersed in an aqueous solution of a boron compound and then heated, thereby producing a lithium-manganese compound oxide. The resulting compound powder of boron compound and manganese oxide decreases in surface activity and is expected to suppress reaction with an electrolyte and therefore to improve the storage characteristic.
The addition of boron, however, reduced the growth of particles and tap density and did not directly translate into high capacity required of a battery alone. Moreover, capacity decreased in the valid potential range when boron was combined with a carbon negative electrode, or the reaction of boron with an electrolyte could not be sufficiently suppressed, depending on the synthesizing conditions. Boron therefore did not satisfactorily improve the storage characteristic.
On the other hand, lithium manganate applied to the positive electrode of the battery with the film casing did not satisfy the expected degree of safety. Specifically, when the battery was repeatedly charged and discharged or left in a charging state at a high temperature, gases were generated in the battery and raised the pressure inside the battery, causing the battery to easily swell. The above gases are presumably ascribable to the decomposition of the electrolyte The swell of the contour of the battery is apt to exceed a space allocated thereto when mounted to an electric apparatus, exerting pressure on surrounding parts. In the worst case, the gases bring about the dangerous burst of the battery.
As stated above, although lithium manganate is a hopeful compound oxide capable of replacing LiCoO2, which is the predominant positive electrode active substance, the conventional battery using lithium manganate has the following problems (1) through (3) left unsolved.
(1) High energy density (high charge-discharge capacity) and high cycle life are not easily compatible.
(2) Storage capacity decreases due to self-discharge.
(3) When the battery with the film casing and using LiMn2O4 is repeatedly charged and discharged or held in a charging state in a high temperature environment, gases presumably ascribable to the decomposition of the electrolyte are generated and cause the battery to swell.
Technical problems relating to the production of a battery and the compatibility of lithium manganate with an electrolyte have been pointed out as the causes of the above problems (1) through (3). Paying attention to the material of the positive electrode itself and the influence of the material, the above problems may be accounted for, as will be described hereinafter.
(1) Compatibility of High Energy Density and High Cycle Life
As for the decrease in capacity ascribable to the charge-discharge cycle, the mean valence of Mn ions varies between trivalence and tetravalence as charge compensation derived from the ingress and egress of Li. As a result, Jahn-Teller distortion occurs in crystal. In addition, Mn is eluted from lithium manganate, or impedance increases as a result of the Mn elution. More specifically, the decrease in capacity ascribable to repeated charge-discharge cycle is brought about mainly by the following causes:
(a) influence of impurities;
(b) elution of Mn from lithium manganate and precipitation of eluted Mn on a negative electrode active substance or on separator;
(c) inactivation ascribable to the isolation of active substance particles;
(d) influence of acids derived from contained water; and
(e) deterioration of an electrolyte ascribable to the release of oxygen from lithium manganate.
Assume that a single spinal phase is formed. Then, Mn is eluted presumably because trivalent Mn partly changes into tetravalent Mn and divalent Mn and causes Mn to be easily dissolved in an electrolyte, and because the relative amount of Li ions becomes short. Presumably, repeated charging and discharging accelerates the generation of irreversible capacity and disturbance to the atomic arrangement of crystal, while causing the eluted Mn ions to precipitate on the negative electrode or on the separator and obstruct the migration of Li ions. Furthermore, the cubic symmetry of lithium manganate is distorted due to the ingress and egress of Li on the basis of the Jahn-Teller effect, resulting in the expansion or contraction of the unit lattice length by several percent. Consequently, the repeated charge-discharge cycle may even make electric contact partly defective or may prevent the isolated particles from functioning as an electrode active substance.
Furthermore, the elution of Mn presumably promotes the release of oxygen from lithium manganate. Lithium manganate with many oxygen defects is increased in 3.3 V Plato capacity and therefore deteriorated in cycle characteristic as the cycle is repeated. Also, the release of much oxygen presumably has adverse influence on the decomposition of an electrolyte and thereby deteriorates the cycle. It has been proposed to solve this problem by, e.g., improving the synthesizing method, adding another transitional element or enriching the Li composition. None of these schemes has succeeded to achieve both of high discharge capacity and high cycle life.
In light of the above, there may be reduced lattice distortion or reduced oxygen defects by way of example.
(2) Decrease in Storage Capacity Due to Self-Discharge
Regarding this problem, let the defective alignment of positive and negative electrodes ascribable to a production process and internal shorting derived from the entry of metal waste be excepted. Then, an improvement in storage characteristic is expected to enhance the stability of lithium manganate, i.e., to reduce Mn to be eluted, reaction with an electrolyte, and release of oxygen.
(3) Swell of Battery Using LiMn2O4 
Presumably, the gases that cause the battery to swell are ascribable to an occurrence that Mn is precipitated on a negative electrode active substance and forms a high-resistance film on the surface or the substance. The high-resistance film is likely to promote the decomposition of an electrolyte on the surface of the negative electrode, resulting in the generation of hydrogen gas. Further, oxygen released from lithium manganate may produce oxygen gas, carbon monoxide gas and carbon dioxide gas on the surface of a positive electrode. Such gases cause the battery with the film casing to swell.
The above-described problems (1) through (3) become more serious in a high-temperature environment, preventing the applicable range from being extended. However, only a limited range of materials are available that implement potential, which satisfies performance required of the state-of-the-art high performance secondary battery, e.g., high electromotive force, flat voltage during discharge, cycle characteristic, and energy density. It is therefore necessary to realize new lithium manganate with spinel structure that obviates the deterioration of capacity ascribable to charging and discharging and has desirable cycle characteristic and storage characteristic.
Japanese Patent Laid-Open Publication No. 10-112318 teaches that a positive electrode active substance is implemented by a mixture of LiMn2O4 or similar lithium-manganese compound oxide and LiNiO2or similar lithium-nickel compound oxide. In accordance with this document, great charge-discharge capacity is achievable because irreversible capacity at the time of initial charging is made up for. Japanese Patent Laid-Open Publication No. 7-235291 also describes that LiCo0.5Ni0.5 O2 is introduced into LiMn2O4 in order to synthesize a positive electrode active substance.
We, however, experimentally found that the mixture of the lithium-manganese compound oxide and the lithium-nickel compound oxide did not satisfy the charge-discharge characteristic, particularly cycle life and capacity storage at high temperatures, or self-discharge characteristic alone. This is because not all the lithium-nickel compound oxides can uptake hydrogen ions, as will be described specifically later. That is, only a particular lithium-nickel compound oxide in accordance with the present invention can effectively obviate the deterioration of a lithium-manganese compound oxide or that of an electrolyte.
Moreover, the mixture of the lithium-manganese compound oxide and the lithium-nickel compound oxide was used as the positive electrode active agent of the battery including the film casing. The mixture, however, had no effect as to the swell of the battery ascribable to the charge-discharge cycle and the storage in a charging state (particularly at high temperatures) alone.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 10-125323, 11-071115, 11-204110 and 11-297361, Japanese Patent Nos. 2512239, 2512241, 2517176, 2547137, 2584123, 2797526 and 2797528, and Japanese Patent Publication Nos. 7-70329, 7-73051 and 7-118317.
It is therefore an object of the present invention to provide a nonaqueous electrolyte type secondary battery with a film casing desirable in battery characteristic, particularly a charge-discharge characteristic and a storage characteristic, and safety and swelling little despite a repeated charge-discharge cycle or storage.
A secondary battery of the present invention includes an electricity generating element, which includes at least a positive electrode implemented by a lithium-manganese compound oxide, a negative electrode, an electrolyte and a separator, and films encasing the electricity generating element is disclosed. The secondary battery further includes a composition causing the electrolyte to react with water to thereby produce hydrogen ions, and a hydrogen ion uptaking agent so positioned as to contact the electrolyte existing in the battery.